Molecular beam epitaxy (MBE) and chemical vapor depositions (CVD) and, more particularly, metal organic chemical vapor deposition (MOCVD) have become the two leading technologies for the deposition of epitaxial layers. These, in turn, are critical to the digital electronics and optical electronic compound semiconductor industry. The use of molecular beam epitaxy, however, while continuing to offer more precise control, is by its nature slow, expensive and difficult to maintain. Therefore, significant efforts have been made to improve MOCVD or organo metallic vapor phase epitaxy (OMVPE) into a more precisely controllable system.
The MOCVD reactors have taken various forms, including horizontal reactors in which the wafer is mounted at an angle to the impinging process gases; horizontal reactors with planetary rotation in which the gases pass across the wafers; barrel reactors; and recently, vertical high-speed rotating disk reactors in which the gas or gases are injected downwardly onto a substrate surface which is rotating within a reactor.
These types of CVD reactors have been found useful for wide varieties of epitaxial compounds, including various combinations of semiconductor single films and multilayered structures such as lasers and LED'S.
The considerable number of these vertical reactors in which the wafers are located on a rotating disk include such reactors as shown in Hitchman et al., "Chemical Vapor Deposition. Principles and Applications," Academic Press, 1993; and Tompa et al., "Design and Application of Large Area RDRs," III-Vs Review, vol. 7, no. 3, 1994. In these reactors, in order to attempt to force the gas mixture to flow uniformly from the top of the reactor towards the wafer carrier, various techniques have been employed. These have included the use of a fine wire mesh, such as that shown in Wang et al., U.S. Pat. No. 4,997,677. In this reactor, the fine stainless steel wire mesh 46', which can also be a sintered coarse frit disk, is intended to more uniformly distribute the gases in this manner. In this case, however, the gases mix in an antechamber before passing through the wire mesh, and reactant gases can react with each other therein before entering the reactor chamber itself. In order to overcome such difficulties, the antechambers have been divided into segments for each reactant gas, for example, as shown in Tompa et al., mentioned above, and in European Patent No. 687,749, in which separate parallel chambers in separate planes carry separate gases which are then fed through individual conduits into the reactor chamber. Pre-reaction nevertheless occurs in such systems after they pass through the wire mesh and/or during flow to the wafer carrier itself. Some of these designs are also quite complicated. In addition, in Bartholomew et al., U.S. Pat. No. 5,136,975, both in the prior art device shown in FIG. 2 and in the injector of the invention shown in FIG. 4, various devices are used in an attempt to prevent the gases from interacting until after they leave a port from a secondary plenum. Similarly, in Watabe, U.S. Pat. No. 5,500,256, spiral gas flow paths are shown in FIG. 1a and FIG. 2 thereof.
Another problem encountered with rotating disk reactors is non-uniformity and temperature distribution. More particularly, it has been found that in the injector plates or other systems used for injection of the gases the center of the injector generally has a higher temperature than the edges connected to the reactor walls, thus resulting in premature decomposition of the wire mesh when it is used, as well as various flow disturbances. In European Patent No. 687,749, mentioned above, a separate coolant chamber 20 is utilized in connection with the various small tubes or conduits through which the gases are flowing. Uniform injector plate cooling cannot be obtained in this type of a device, which does not include provisions for forcing the water to flow around each of the tubes, for example. Also, in Bartholomew et al., discussed above, a cooling plate 72 is employed surrounding chute 96 from which the gases are injected into the reactor.
Another factor in connection with these rotating disk reactors relates to the relationship between the size of the reactor and the size of the rotating wafer carrier or substrate carrier plate. Thus, it is normally natural for the reactor to be larger in order to pump reactant therethrough. However, this, in turn, requires additional consumption of reactants, resulting in lower efficiency. On the other hand, in those reactors in which the inlet surface is smaller than that of the wafer carrier with a gradually increasing diameter, depletion effects cannot be eliminated. These include, for example, Sato et al., U.S. Pat. No. 5,344,492, as well as Fotiadis et al., "Complex Phenomena in Vertical MOCVD Reactors: Effects on Deposition Uniformity and Interface Abruptness," General Crystal Growth 85 (1987), pp. 154-164. Also, in Kennedy, U.S. Pat. No. 5,173,336, flows guides 52 are shown in FIG. 3 thereof for preventing portions of the chemical vapor flow from being deflected back towards the vapor source to suppress recirculation therein. In Fujaimura, U.S. Pat. No. 5,024,748, prior art references are discussed, such as that shown in FIG. 2, which use reflectors 13 for directing plasma onto a substrate in a microwave plasma processing apparatus.
There are also a number of different techniques which are employed for heating the wafer carriers in these reactors. Radiant heating elements are installed below the wafer carrier, such as those shown in the Hitchman et al. and Tompa et al. articles discussed above. In these types of devices, the heating assembly, however, is open to the reactor environment. This can cause various effects including reducing the life, time and repeatability of these processes. For this reason, in reactors such as those shown in the Fotiadis et al. article and in European Patent No. 687,749 discussed above, a heating assembly has been installed inside a rotating shell so that inert gases purged through the shell can provide a protective environment. In these reactors, the rotating shell is usually sealed by connection with a hollow shaft rotary vacuum feed-through having a diameter which is less than that of the heating elements, which cannot, therefore, be removed through the hollow shaft feed-through. Thus, the heating systems in these reactors are generally provided in a system in which they cannot be replaced or serviced without opening the reactor to the atmosphere. Also, in Stitz, U.S. Pat. No. 4,607,591, a lamp enclosure for heating lamps is included in a plasma-enhanced CVD reactor.
Finally, in these types of reactors, various systems are provided for rotating and sealing these reactor drive mechanisms. Most commonly, ferro-fluidic types of vacuum rotary feed-throughs are used to seal the rotating shafts, and these generally have a low temperature limit of below 100.degree. C. Thus, complex and expensive cooling systems are required therefor. Some cooling systems are shown in Miagi, U.S. Pat. No. 5,421,892, and Tsuga, U.S. Pat. No. 4,771,730.
It is therefore an object of the present invention to provide a simple injector plate design for rotating disk reactors which can be scaleable to large diameters and which can introduce the various gases and/or reactants and maintain them separately within the reactor up to or close to the mixed gas thin boundary layer just above the wafer carrier itself.
It is yet another object of the present invention to design an injector plate which can also have a controllable uniform temperature profile.
It is yet another object of the present invention to develop a confined inlet reactor whose diameter is equal or close to that of the wafer carrier itself.
It is yet another object of the present invention to provide an inlet reactor design in which the diameter extension near the wafer carrier is designed to provide for higher deposition efficiency without depletion effects.
It is yet another object of the present invention to design a rotating disk reactor using a rotating shell in which the heating assembly is located within the shell and is purged by gases or pumped to a vacuum.
It is yet another object of the present invention to provide such a rotating disk reactor using a rotating shell which has a diameter equal to or larger than that of the heating assembly so that the heating assembly can be removed and different heating assemblies can be substituted therefor without altering the reactor design or opening the reactor to the atmosphere.
It is yet another object of the present invention to design a rotating disk reactor employing a rotating shell design with a simple and efficient cooling system for the rotating shaft.